diff --git a/crates/benilla-formats/src/chain.rs b/crates/benilla-formats/src/chain.rs
index 4fd5d238f..8974ae524 100644
--- a/crates/benilla-formats/src/chain.rs
+++ b/crates/benilla-formats/src/chain.rs
@@ -59,6 +59,19 @@ fn is_patch_glob_match(name: &str) -> bool {
 /// [`VANILLA_BASE_ORDER`] archives, `patch.MPQ`, every `patch-?.MPQ` by case-folded name (the
 /// reference sorts descending with `strnicmp` and walks backwards), then `speech2.MPQ`. Names
 /// match case-insensitively and come back as found on disk.
+/// `$BENILLA_STOCK_CHAIN` set and not empty: a `Data` directory mounts only the stock 1.12.1
+/// archives, its base set, `patch.MPQ`, `patch-2.MPQ` and `speech2.MPQ`, and leaves out every
+/// other `patch-?.MPQ` (texture packs, interface mods), so the tests that read the install see the
+/// data they assert. The language pack's folder is not filtered. `scripts/unit-tests.sh` sets it.
+fn stock_chain_only() -> bool {
+    std::env::var_os("BENILLA_STOCK_CHAIN").is_some_and(|v| !v.is_empty())
+}
+
+/// Whether `name` is a stock 1.12.1 patch archive.
+fn is_stock_patch(name: &str) -> bool {
+    name.eq_ignore_ascii_case("patch-2.MPQ")
+}
+
 fn mount_order(dir_names: &[String]) -> Vec<String> {
     let find = |want: &str| {
         dir_names
@@ -86,11 +99,17 @@ impl Chain {
     /// `"Failed to open archive"` and goes on, because a skipped corrupt archive surfaces only as
     /// missing files far downstream.
     pub fn open(path: &Path) -> Result<Self> {
-        Self::open_without(path, &[])
+        Self::open_dir(path, stock_chain_only(), &[])
     }
 
     /// [`Chain::open`], `Data`'s archives named in `skip` left out.
     fn open_without(path: &Path, skip: &[String]) -> Result<Self> {
+        Self::open_dir(path, stock_chain_only(), skip)
+    }
+
+    /// [`Chain::open`]; `stock` keeps the `Data` directory to its stock archives
+    /// ([`stock_chain_only`]).
+    fn open_dir(path: &Path, stock: bool, skip: &[String]) -> Result<Self> {
         let mut archives = Vec::new();
         if path.is_dir() {
             let mut names: Vec<String> = std::fs::read_dir(path)
@@ -105,7 +124,9 @@ impl Chain {
             // read_dir order is arbitrary; sort so case-variant ties resolve deterministically.
             names.sort();
             for name in mount_order(&names) {
-                if skip.iter().any(|s| s.eq_ignore_ascii_case(&name)) {
+                if skip.iter().any(|s| s.eq_ignore_ascii_case(&name))
+                    || (stock && is_patch_glob_match(&name) && !is_stock_patch(&name))
+                {
                     continue;
                 }
                 let mpq = path.join(&name);
@@ -161,7 +182,9 @@ impl Chain {
             let folder = pack
                 .pack_folder(path)
                 .context("ruRU language pack is not installed")?;
-            chain.archives.extend(Self::open(&folder)?.archives);
+            chain
+                .archives
+                .extend(Self::open_dir(&folder, false, &[])?.archives);
         }
         chain.locale = Locale::for_language_pack(pack);
         Ok(chain)
@@ -473,6 +496,54 @@ mod tests {
         );
     }
 
+    /// The stock chain keeps the base set, `patch.MPQ` and `patch-2.MPQ`, and drops the other
+    /// `patch-?.MPQ` in any case; without it every patch mounts.
+    #[test]
+    fn the_stock_chain_mounts_only_the_stock_patches() {
+        let dir = std::env::temp_dir().join(format!(
+            "benilla-chain-stock-{}-{:?}",
+            std::process::id(),
+            std::thread::current().id()
+        ));
+        let _ = std::fs::remove_dir_all(&dir);
+        std::fs::create_dir_all(&dir).unwrap();
+        for archive in [
+            "interface.MPQ",
+            "patch.MPQ",
+            "patch-2.MPQ",
+            "patch-A.mpq",
+            "patch-F.MPQ",
+        ] {
+            let line = format!("X\\{archive}\r\n");
+            std::fs::write(
+                dir.join(archive),
+                fixture_mpq(&[("(listfile)", line.as_bytes())]),
+            )
+            .unwrap();
+        }
+        let stock = Chain::open_dir(&dir, true, &[])
+            .expect("open")
+            .enumerate_archive_files("x\\");
+        let every = Chain::open_dir(&dir, false, &[])
+            .expect("open")
+            .enumerate_archive_files("x\\");
+        let _ = std::fs::remove_dir_all(&dir);
+        assert_eq!(
+            stock,
+            ["X\\patch.MPQ", "X\\patch-2.MPQ", "X\\interface.MPQ"]
+        );
+        assert_eq!(
+            every,
+            [
+                "X\\patch.MPQ",
+                "X\\patch-2.MPQ",
+                "X\\patch-A.mpq",
+                "X\\patch-F.MPQ",
+                "X\\interface.MPQ"
+            ]
+        );
+    }
+
     /// An archive left out by name, in any case, does not mount; the others keep their order.
     #[test]
     fn a_left_out_archive_does_not_mount() {
@@ -491,7 +562,7 @@ mod tests {
             )
             .unwrap();
         }
-        let chain = Chain::open_without(&dir, &["PATCH-3.mpq".to_owned()]).expect("open");
+        let chain = Chain::open_dir(&dir, false, &["PATCH-3.mpq".to_owned()]).expect("open");
         let walked = chain.enumerate_archive_files("x\\");
         let _ = std::fs::remove_dir_all(&dir);
         assert_eq!(
diff --git a/scripts/unit-tests.sh b/scripts/unit-tests.sh
index bf822323c..f31d204aa 100755
--- a/scripts/unit-tests.sh
+++ b/scripts/unit-tests.sh
@@ -7,6 +7,9 @@
 #   scripts/unit-tests.sh --workspace
 #   scripts/unit-tests.sh -p benilla-app --no-default-features --lib
 set -euo pipefail
+# The tests that read the install assert the stock 1.12.1 data: the chain leaves out any other
+# patch archive a player added (texture packs, interface mods) (`Chain::open`).
+export BENILLA_STOCK_CHAIN=1
 if cargo nextest --version >/dev/null 2>&1; then
     exec cargo nextest run --no-fail-fast --no-tests=pass "$@"
 fi